In his new post on PHPEverywhere today, John Lim shares some of the experiences he's had so far in making the move up from PHP 4 to PHP 5 in his applications.
The transition was relatively painless. [...] What's nice about PHP5 is that it caught some errors that have been lingering in our code: PHP5 no longer allows a function to be defined twice, and some basic variable referencing errors that we missed previously.
They made the move to PHP5 for the latest versions of their applications, but have still stuck with the legacy, PHP4 versions for the time being to give customers a buffer period to make the move themselves. He also mentions changes to the way they make Ajax calls. John is a lead developer for both the ADOdb and PHPLens projects.